Angiogenesis is critical for the development and maintenance of glioblastomas, the most malignant and most common form of primary brain tumors. Current evidence indicates that recruitment of tumor vessels from normal surrounding tissue, and development and maintenance of tumor angiogenesis require a delicate balance between the timing and level of expression of two major angiogenesis factors: Angiopoietin-2 (Ang-2) and the vascular endothelial growth factor-A (VEGF-A). The combination of Ang-2 and VEGF-A expressions results in loosening of peri-endothelial support rendering the newly exposed endothelial cells able to multiply. We hypothesize that there is a regulatory signaling loop involving the coordinated and sequential expression of VEGF and Ang-2, and that elucidating the underlying mechanisms can lead to developing a better model for the angiogenic process that occurs in human gliomas. Importantly, we have uncovered the expression of Tie2 in glioma cells. Moreover, exploiting the interdependence between VEGFand Ang-2 could be used to develop more effective and rational anti-angiogenesis therapies for brain tumors than currently exist. Therefore, in this application we shall aim to uncover the mechanisms of interaction between Ang-2 and VEGF-A, the role of Ang-2 expression in a dynamic tumor model of glioma angiogenesis, and to develop an effective treatment based on the simultaneous targeting of Ang-2, using conditionally-replicative adenoviruses, and direct down modulation of VEGF using antisense strategies. The specific aims are as follows: Specific Aim 1: To characterize the modulation of VEGF-A by Ang-2; Specific Aim 2: To determine the effects of Angiopoietin-2 on the growth kinetics and angiogenesis development in a glioma model; and Specific Aim 3: To examine the anti-glioma effect of combined therapies based on the transfer of Angiopoietin-2, using oncolytic adenoviruses, and anti-sense VEGF. This project should yield important mechanistic information about the abnormal regulation of angiogenesis in gliomas. Furthermore, data obtained from our studies will hopefully constitute a rational basis for the development of a Phase 1/11clinical trial to test the toxicity and efficacy of a triple treatment for malignant gliomas, which combines the overexpression of Ang-2, decreased VEGF expression, and oncolysis.